The distinction between the corner radius and the effective turning radius is crucial and often overlooked. The corner radius may be a simple or a complex curve and depends primarily on the presence of on-street parking, bike lanes, the number of travel lanes, medians, and traffic control devices.

Designers often determine corner radii based on the intersection geometry only and overlook the effective radius. As a result, drivers making a turn on a green signal have little incentive to turn into the nearest receiving lane and routinely turn as wide as possible to maintain travel speeds.

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Recommended

Turning speeds should be limited to 15 mph or less. Minimizing turning speeds is crucial to pedestrian safety, as corners are where drivers are most likely to encounter pedestrians crossing in the crosswalk.3

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Click to enlarge.

Minimize effective turning radius where possible by employing one or more of the following techniques:

Optional

In cases where the curb radius of a given intersection has resulted in an unwieldy or unsafe crossing distance, but where funding is not available to reconstruct the curb immediately, a city may delineate the appropriate curb radius using interim materials such as epoxied gravel, planters, and bollards. This should be a temporary option until funding becomes available for a more permanent treatment.5

Narrower streets with curbside travel lanes may require larger corner radii because the effective turning radius mirrors the actual corner radius. The same holds true for streets with curb extensions. Streets should not be designed with larger corner radii in anticipation of the entire roadway being used for vehicle traffic at some point in the future.

Footnotes

A literature review of the topic of curb radius and injury severity at intersections points out that “larger radii are less safe for bicycles and pedestrians because they allow for higher vehicle speeds through the turn and result in larger crossing distances.”